Seismic Design of Lightly Reinforced Concrete Walls

Abstract

During the 2010/2011 Canterbury earthquakes in New Zealand, several lightly reinforced concrete (RC) walls in multi-storey buildings formed only a limited number of cracks at the wall base with fracture of vertical reinforcement observed. Initial investigation highlighted that the vertical reinforcement content was a key parameter that influenced the cracking behaviour and ductility of lightly reinforced concrete walls. A combination of large-scale experimental testing and numerical modelling was used to investigate the seismic behaviour of RC walls with minimum vertical reinforcement subjected to simulated earthquake loading. A series of experimental tests highlighted that the minimum distributed vertical reinforcement requirements for RC walls in New Zealand Concrete Standard NZS 3101:2006 (Amendment 2) are insufficient to ensure that a large number of secondary cracks form in the plastic hinge region. A finite element model was developed that accurately captured the global and local behaviour of lightly reinforced concrete walls that was observed during the test. From the results of finite element analyses, the cracking behaviour and drift capacity of RC walls with the minimum distributed vertical reinforcement in NZS 3101:2006 (Amendment 2) would be further influenced by wall size, reinforcement properties, and concrete strength. The experimental and numerical modelling results both showed the minimum distributed vertical reinforcement requirements in NZS 3101:2006 (Amendment 2) are only suitable for walls designed for low ductility demands. During the course of this research, new amendments were proposed to the minimum vertical reinforcement requirements for limited ductile or ductile plastic regions of RC walls in NZS 3101:2006 (Amendment 3 draft). A second series of laboratory tests confirmed that the additional vertical reinforcement limits proposed for the end region of ductile walls in NZS 3101:2006 (Amendment 3 draft) are sufficient to ensure that well distributed secondary cracks occurred in the plastic hinge region and are suitable for limited ductile and ductile walls. The requirements for minimum vertical reinforcement in RC walls from different concrete and seismic design standards worldwide were reviewed and compared to establish the key differences between these alternative requirements. A comprehensive study on the behaviour of walls with minimum vertical reinforcement requirements in accordance with current concrete design standards was conducted using the developed finite element model. The model results indicated that the minimum vertical reinforcement requirements in most concrete standards are insufficient to ensure desirable seismic performance for ductile RC walls. Recommendations are provided related to minimum vertical reinforcement requirements for current concrete standards. To address the deficiencies of existing requirements, new theory and equations were developed to determine the required minimum vertical reinforcement for RC walls of different ductility classes considering key parameters. The proposed formulas were verified against experimental data and numerical modelling results. The comparison with other requirements for minimum vertical reinforcement in existing concrete design standards showed the superiority of the proposed requirements. The analyse results of this research was used for justifying the proposed revisions of minimum vertical reinforcement limits to NZS 3101 and also, they can be used as a basis for assessment of seismic behaviour of lightly reinforced concrete walls and revisions of minimum vertical reinforcement limits in other concrete standards.